Infrared (IR) signaling with the aid of night vision and/or thermal imaging technology for detection has provided a means towards relatively covert free-space signaling and communications applications. In addition, IR signaling can serve as a means of free-space communications within the various atmospheric windows, and can denote a change in sensor status (e.g. the detection of chemical agent).
However, current approaches use either high-power IR lasers (FIG. 1A) or spectrally broad, highly diffuse emitters (FIG. 1B). As can be seen from FIG. 1A, in the case of high-power IR lasers, the high power of the laser produces a high-visibility signal under the appropriate imaging conditions; however such laser-based sources tend to be highly directional, such that outside of a narrow cone of angles between the detector and the source, an optical signal from such a high-power laser will not be readily observed. The high output power can also cause a “halo” effect whereby the detected signal saturates multiple pixels washing out part of the image. In addition, such systems have a large power requirement that limits battery life, making them unsuitable for many field uses.
In the case of diffuse emitters as shown in FIG. 1B, the source may be observed over a broader range of angles; however, this comes at the cost of being spectrally broad and weak in amplitude, making their observation outside of close ranges difficult. In addition, because they are spectrally broad, such emitters may clearly advertise the location of the source, since they may be observable not only to authorized observers having thermal imagers in a specified wavelength range, e.g., 3 μm thermal imagers, but also to anyone having conventional near-IR night vision goggles. Thus, these sources are very easy to replicate and intercept.
Thus, despite substantial advancements in technology, significant issues with these applications persist. To ameliorate these issues, new technological approaches are required.
Polar dielectric crystals experience an imbalance of the partial ionic charges of the atomic species in the crystal. For example, in the exemplary silicon carbide polar dielectric illustrated by the block diagram in FIG. 2A, a charge imbalance exists between the partial positive charge “δ+” of the Si ions in the lattice and the partial negative charge “δ−” of the C ions. The presence of this partial ionic charge imbalance enables stimulation of surface phonon polaritons (SPhPs) in such polar dielectric materials. In addition, the sub-diffractional confinement of light can be observed using metallic and highly doped semiconductor species (including many of the polar dielectric SPhP crystals when a high density of free carriers are present), providing similar behavior in the higher frequency regimes. In these cases, the incident light couples to free electrons or holes (“carriers”) in the material in a manner illustrated by the block schematic in FIG. 2B, providing the mechanism for the sub-diffractional confinement of light.
Incident light at wavelengths corresponding to frequencies ω between the frequency ωTO of the transverse optic (TO) and the frequency ωLO of the longitudinal optic (LO) phonons of a polar dielectric material, e.g., as shown by 4H—SiC Raman spectrum curve 301 shown in FIG. 3A, induces coherent oscillations of the crystal lattice of the material. Because of the presence of the positive and negative atomic charges δ+ and δ−, these oscillations induce a large surface electromagnetic field that causes a normally transparent dielectric to become highly reflective within this spectral band, referred to as the “Reststrahlen” band, as seen by dashed IR reflectance curve 302 for 4H—SiC shown in FIG. 3A (and also shown in FIG. 3B). See Joshua D. Caldwell, Lucas Lindsay, Vincenzo Giannini, Igor Vurgaftman, Thomas L. Reinecke, Stefan A. Maier and Orest J. Glembocki, “Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons,” Nanophotonics 2015; 4: 44-68. Correspondingly, the real part of the dielectric function (permittivity) becomes negative, as shown by solid curve 303 shown in FIG. 3B, which enables the confinement of resonant light within sub-diffractional volumes through the nanostructuring of these polar dielectrics or at interfaces and surfaces of such materials.
A plot illustrating the wide range of surface plasmon (NIR to MWIR) and surface phonon (MWIR to FIR) polariton materials, often referred to collectively as “polaritonic” materials, is provided in FIG. 4, with standard plasmonic metals (e.g. silver, gold, aluminum and copper) that support surface plasmons in the ultraviolet and visible and other more exotic types of polaritons (e.g. exciton polaritons) being omitted for simplicity; however, one skilled in the art will readily recognize that the materials shown in FIG. 4 are merely exemplary and by no means constitute an exhaustive and complete list of available polaritonic materials.
It was recently demonstrated by researchers that the Naval Research Laboratory (NRL) that nanoscale structures fabricated out of silicon carbide (SiC) and hexagonal boron nitride (hBN) result in spectrally narrow resonances within the mid-infrared (10.3-12.5 um for SiC; 6.2-7.3 um and 12.1-13.2 um for hBN), with resonance linewidths as narrow as 3 cm−1, on par with well-defined crystal vibrations. See Joshua D. Caldwell, Orest J. Glembocki, Yan Francescato, Nicholas Sharac, Vincenzo Giannini, Francisco J. Bezares, James P. Long, Jeffrey C. Owrutsky, Igor Vurgaftman, Joseph G. Tischler, Virginia D. Wheeler, Nabil D. Bassim, Loretta M. Shirey, Richard Kasica, and Stefan A Maier, “Low-Loss, Extreme Subdiffraction Photon Confinement via Silicon Carbide Localized Surface Phonon Polariton Resonators,” Nano Lett. 2013, 13, 3690-3697 (SiC); and Joshua D. Caldwell, Andrey V. Kretinin, Yiguo Chen, Vincenzo Giannini, Michael M. Fogler, Yan Francescato, Chase T. Ellis, Joseph G. Tischler, Colin R. Woods, Alexander J. Giles, Minghui Hong, Kenji Watanabe, Takashi Taniguchi, Stefan A. Maier, and Kostya S. Novoselov, “Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride,” NATURE COMMUNICATIONS (2014) 5:5221 (hBN). See also U.S. Pat. No. 9,195,052 to Long et al, entitled “Actively Tunable Polar-Dielectric Optical Devices” (Nov. 24, 2015); U.S. Pat. No. 9,244,268 to Long et al., entitled “Actively Tunable Polar-Dielectric Optical Devices” (Jan. 26, 2016); U.S. Pat. No. 9,274,532 to Long et al., entitled “Actively Tunable Polar-Dielectric Optical Devices” (Mar. 1, 2016); and U.S. Patent Application Publication No. 2016/0103341 by Long et al.
These nano-scale polaritonic structures, such as the SiC bowtie antenna arrays whose reflectances are illustrated by the plots shown in FIG. 5A, can provide passive polarization- and frequency-selective infrared reflection spectra due to the sub-diffractional resonance modes supported within the nano-scaled structures. In addition, the IR emission spectra of such SiC bowtie antenna arrays at T=350° C. shown in FIG. 5B demonstrate that by heating the structures to modest temperatures, tailored IR emission can be produced, with the emission retaining the polarization and narrow spectral bandwidth of the resonances observed in the reflection spectra shown in FIG. 5A. It has been observed within our lab that heating even to small temperatures such as 50° C. is sufficient to induce a measurable emission.
This phenomenon was originally demonstrated for SiC micron-scale gratings and microwires. See Jean-Jacques Greffet, Rémi Carminati, Karl Joulain, Jean-Phillipe Mulet, Stéphane Mainguy, and Yong Chen, “Coherent emission of light by thermal sources,” Nature 2002, 416, 61-64; and Jon A. Schuller, Thomas Taubner, and Mark L. Brongersma, “Optical antenna thermal emitters,” NATURE PHOTONICS, Vol. 3 (November 2009), pp. 658-661. If a device comprising a plurality of nano-scale emitters is fabricated on a substrate homogeneous with the nano-scale emitters (e.g. SiC bowties on a SiC substrate), the IR emission from the nano-scale emitters will be superimposed upon the broadband high reflectivity (low emission) of the underlying substrate, providing a large optical contrast. On the other hand, if the nano-scale structures are fabricated (or grown) on a dissimilar substrate material, the IR emission from those structures will be superimposed upon the IR emission of the underlying substrate and will result in a broad gray-body radiation spectrum with the narrow-band IR emission signature from the nano-scale structures superimposed thereon.
While the IR emission from localized SPhP resonators discussed in the literature has primarily focused on SiC structures, see Greffet, supra, and Schuller, supra, in principle, any polar dielectric crystal can be used, provided the Reststrahlen band is in an appropriate frequency range for IR emission at the temperature of operation. This is equally applicable to surface plasmon polaritons. In the case of the latter, such materials will operate over a broader spectral range, but the resonance linewidths will be significantly broadened with respect to the lower-frequency SPhP materials.
Although use of most plasmonic metals (e.g. gold and silver) would be cost-prohibitive and would require excessive temperatures, even in excess of their melting points, to achieve emission near their resonances in the visible spectral region, a significant effort has also been focused on developing alternative lower-loss plasmonic materials. For instance, developments from Prof. Jon-Paul Maria's group at North Carolina State University have led to a low-loss plasmonic material in the form of dysprosium-doped cadmium oxide that would offer the potential for IR emitters in the 2-8 μm range. See E. Sachet, C. T. Shelton, J. S. Harris, B. E. Gaddy, D. L. Irving, S. Curarolo, B. F. Donovan, P. E. Hopkins, P. A. Sharma, A. L. Sharma, J. F. Ihlefeld, S. Franzen, and J.-P. Maria, “Dysprosium-doped cadmium oxide as a gateway material for mid-infrared plasmonics” Nature Materials 14, 414-420 (2015).
Additional materials such as transparent conducting oxides would provide opportunities in the 1-5 um region. See Gururaj V. Naik, Vladimir M. Shalaev, and Alexandra Boltasseva, “Alternative Plasmonic Materials: Beyond Gold and Silver,” Adv. Mater. 2013, 25, 3264-3294. While the optical losses (efficiency) of these plasmonic materials is higher than in their phonon polariton counterparts, which will result in broader emission linewidths, they do offer the potential for tailored IR emitters in a spectral range where currently no known phonon polaritons exist (λ<6 μm). As noted above, a wide array of these polaritonic materials is presented in FIG. 4. It should be noted that all of those presented are in various states of commercial maturity, but successful synthesis of all has been demonstrated. Further, it should be stated that this list is not meant to be exhaustive, but that this approach is equally applicable to tailored IR emitters of all kinds, for instance SPhP, surface plasmon and dielectric resonators.